1. Field of the Invention
This invention relates to an image forming apparatus or nonimpact printer that uses an ink or dye ribbon and a form of energy to cause the transfer of a selected portion of ink or dye to a substrate to form an image, and particularly to such printers that are capable, e.g., of carrying out both thermal transfer and dye diffusion printing. Specifically, the invention relates to a printer in which separate print heads, ribbons and image transfer mechanisms are provided to permit multiple image transfers onto a substrate that remains held throughout to help ensure accurate image registration.
2. Background Information
In thermal transfer printing, an ink- or dye-bearing ribbon is pressed against a substrate between the thermal print head and a platen. The substrate can be ordinary or specially coated paper, or also plastic film, acetate and the like. Resistor elements in the print head are selectively subjected to a heating current to cause the transfer of ink or dye from the ribbon to the substrate in a desired pattern. In the analogous elecroresistive printing process, the print head incorporates needle-like electrodes that are selectively brought into contact with an electroresistive ribbon layer to cause an electrical current to pass therethrough, through an underlying electrically conductive layer, and thence back to a return electrode placed against the electroresistive layer. Variously colored heat sensitive ink layers are disposed on the side of the conductive layer opposite the electroresistive layer, and the heat generated within the electroresistive layer passes through the conductive layer to cause softening or melting and thus transfer of the ink in the ink layer to the substrate in a desired pattern. In dye diffusion printing, the diffusion or sublimation of dye from the ribbon to the substrate is employed instead of melting ink. There also exist direct energy processes, and other image transfer mechanisms employ combinations of chromogenic materials and encapsulated radiation curable composition, combinations of a developer and a photosensitive microencapsulated material, materials subject to transfer when acted upon by light, and materials in which a combination of light and heat will bring about changes in such physical parameters of said materials as viscosity or various softening, melting and glass transition temperature.
For black printing, a single black ink or dye panel can be used, and a single ribbon might include a multiplicity of black panels, each of the same size (e.g., letter, legal size, A-4 or A-5, etc.) as the substrate onto which the image is to be transferred. For color printing, generally thee separate color panels in the colors yellow, magenta, and cyan are used, and these panels will typically be angled in repetitive units along the ribbon. A full color image is formed from the three primary colors by printing one over the other, typically in the order yellow, magenta and cyan.
The three colors can also be superimposed to produce a black image, but such an image will not be of the same quality as is produced using a single black ink or dye. To use three color panels to produce a black image is also wasteful of color panels, and significantly increases the time required for the image transfer. To obtain the higher quality black image available from a black ribbon has hitherto required replacement of the three-color ribbon with a black ribbon, hence to avoid such repetitive ribbon changing, ribbons have been devised that include black panels in addition to the thee color panels. Also, the present applicant's U.S. patent application Ser. No. 08/039,871 filed Mar. 30, 1993 entitled COMBINATION INK OR DYE RIBBON AND APPARATUS FOR NONIMPACT PRINTING describes "combination ribbons" which, in addition to three-color and black panels, also include different types of panels, e.g., both thermal transfer and dye diffusion, that may be used for different printing requirements. It thus becomes possible to carry out various kinds of printing using just one ribbon.
One disadvantage of the foregoing procedure, however, is that of needing to traverse through the ribbon in order to arrive at a black ink panel, move back again to carry out color printing, then go back to the black panel, or to another panel type, and so on. Such a process is wasteful of time and can ultimately be damaging to the ribbon. Moreover, the relative amounts of full color and black or other printing (e.g., thermal transfer and dye diffusion) required may not coincide with the relative numbers of different panel colors or types on the ribbon, so that some portions of the ribbon may become wasted.
Another disadvantage of the "combination ribbon" approach of U.S. Pat. application Ser. No. 08/039,871 filed Mar. 30, 1993 noted above is that thermal transfer and dye diffusion printing are generally employed for different purposes. For example, conventional thermal transfer processes, while less restrictive than dye diffusion in terms of the nature of the substrate, do not produce the true continuous tone images that can be obtained using dye diffusion, and it would be difficult to estimate in advance, in developing a ribbon to be used for both of those processes, the relative numbers of ink and dye panels to include on the ribbon. More recent thermal transfer techniques that incorporate variable dot size provide higher quality, but require special ribbons and print heads. It is preferable, therefore, to provide a printer in which the thermal transfer (conventional or more recent) and dye diffusion processes are carried out using different ribbons, each designed for use in just one of those two processes. Of course, if electroresistive printing is also to be employed, that process must likewise have a ribbon therefor. It may also be desired, of course, to provide means for direct energy printing in which no ribbon is employed.
One solution of the foregoing problem has been to employ two printers: one having a three-color ribbon for full color printing, and other a totally black ink ribbon for printing in black. However, that process introduces extra expense, and also requires additional desk space. Moreover, for images that require black printing along with full color printing, to obtain the higher quality black image form the black ink printer it becomes necessary to transfer the substrate onto which the image is to be placed from one printer to the other, and it may be difficult or impossible to obtain the same positioning of the substrate in the one printer as the other, so that the image transferred in the second printing will appear where desired relative to the first image. The alternative procedure of using just one printer but changing ribbons for each kind of printing process, while saving of expense and desk space, introduces even greater operator inconvenience.
In the copier art, U.S. Pat. No. 4,783,681 issued Nov. 8, 1988 to Tanaka et al, describes a system that includes a plurality of copier units, each of which will record an image on a separate recording material when used alone, but when the units are coupled together a single material can receive a first image from a first copier unit and then be transported to a second unit to receive a second, overlaid image, and so on. The plurality of identical recording units are designed to plug one into the other so that recording material can be passed therebetween, and control of such a sequence of recording units is then carried out from the first unit in the sequence. Registration between successive images is controlled firstly by using sheet discharging means within a sheet transferring unit to place a sheet of image-receiving material into the next unit in the sequence, and secondly by transmitting from the transferring unit to that next receiving unit a timing signal to control the registration rollers of the latter unit. Although this Tanaka et al. patent does address the multi-image registration problem noted above, it does not address the transfer of images either by direct energy or by using a transfer material disposed on a ribbon.
Another type of thermal transfer printer employs a back roller instead of a platen such that the back roller faces against the thermal head with the ink donor sheet or ribbon and the recording sheet or receiving medium pressed therebetween as usual, wherein the back roller is driven by a motor so as to advance both the ribbon and receiving medium. U.S. Pat. No. 4,495,507 issued Jan. 22, 1985 to Moriguchi et al. describes such a device that contains two complete "recording stations," i.e, two separately functioning back rollers, back roller drive motors, thermal heads, and ribbons (on associated supply and takeup rolls) wherein the two ribbons provide two-color printing onto a receiving medium that is made to pass first through one recording station and then through the other. This particular "two-station" design has its own type of registration problem, namely, that any difference in diameter between the two back rollers will cause differences in the rate of advance of the receiving medium in the two cases, thereby introducing a color shift in the transferred image. The Moriguchi et al, '507 patent seeks to resolve that problem by using different roller drive motors so that one motor can compensate for such variations in the rate of medium advance.
In two-station printing, it had also been the practice for the image data for the second station in line to be stored in memory, to be released at a time determined by the distance between printing stations and the rate of movement of the ribbon and substrate. Because of the distance of separation required between the two thermal heads in such a linear design, however, the amount of memory required for such data storage came to be a problem, hence a variant of the Moriguchi et al. '507 device, found in U.S. Pat. No. 4,385,302 issued May 24, 1983 to Moriguchi et al., employs a plurality of thermal head assemblies arranged radially around a single backing roller. In a linear, three-station embodiment, synchronization between the image-forming processes is achieved by way of synchronization marks that, along with the first image transfer (e.g., a yellow color transfer), are placed by the first thermal head along one edge of the substrate and are then read by first and second marker sensors associated respectively with second and third thermal head assemblies. In U.S. Pat. No. 4,408,212 issued Oct. 4, 1983 to Moriguchi et al., positional displacement between images in a two- or multiple-head device is avoided by means of a belt that extends around adjacent rollers in contact with the substrate upon which such images are placed so as to eliminate positioning errors arising from any differences in the diameters of such rollers. U.S. Pat. No. 4,410,897 issued Oct. 18, 1983 to Moriguchi et al. describes two separate recording stations of the back roller type that are disposed on opposite sides of the substrate onto which the image is to be transferred so as to permit printing on both sides of the substrate in a single pass.
U.S. Pat. No. 4,462,704 issued Jul. 31, 1984 to Karata et al. sets forth pulse generator means that operate from a single power source for driving a plurality of thermal print heads in the Moriguchi et al. '897 configuration. Similarly, U.S. Pat. No. 4,811,036 issued Mar. 7, 1989 to Gaskill et al. describes two separate printing means, e.g., conventional thermal print heads using separate platens and printing films (i.e., ribbons), disposed on opposite sides of a continuous strip and controlled in such a way that the longitudinal positioning of the images produced by the two thermal print heads on the opposite sides of the continuous strip are coordinated.
U.S. Pat. No. 4,595,303 issued Jun. 7, 1986 to Kuzuya et al. describes a printing apparatus including two assemblies in which one provides character type elements and the other produces characters using a dot matrix. U.S. Pat. No. 4,863,297 issued Sep. 5, 1989 to Fujii describes a thermal printer having three thermal heads, each of which is disposed to transfer ink from a separate colored ribbon. It is not possible from any of these constructions, however, to provide all of the fundamentally different types of printing, i.e., thermal transfer, dye diffusion, and electroresistive, that may be needed in the more modern printing environment.
U.S. Pat. No. 4,815,872 issued Mar. 28, 1989 to Nagashima seeks to describe apparatus having two print heads and associated ribbons and platens: one print head is used conventionally for multicolor printing, and a second print head for which the associated ribbon contains only transparent ink serves to print a protective layer over the multicolor image. This patent thus does not explicitly address the aforesaid registration problems.
Some of the problems remaining within the foregoing art have been addressed by this applicant's application Ser. No. 08/047,144 filed Apr. 12, 1993. Specifically, although application Ser. No. 08/039,871 filed Mar. 30, 1993 made possible the use of various technologies within a single printer by way of "combined ribbons" and multiple printing devices, in application Ser. No. 08/047,144 filed Apr. 12, 1993 is still remained necessary, in order to carry out those different technologies, to move the substrate from one printing location to another. That is, in any of the embodiments of the device described in application Ser. No. 08/047,144 filed Apr. 12, 1993 that have two different print heads that can be placed individually into cooperation with one and the same ribbon/printing system, there is available within a single such system only a single ribbon. Whatever type of printing may be desired must then be provided either (1) by way of panels that are incorporated within that system; or (2) by moving the substrate to a different ribbon/printing location, thereby introducing possible registration errors (even though such errors are indeed minimized by use of the belt and clamp mechanism shown in FIG. 7 of application Ser. No. 08/047,144 filed Apr. 12, 1993). Although the two aforesaid patent applications taken together provide means for a very wide variety of printing tasks, some of the print head/ribbon combinations that would be required may be limited in scope: if but a single printing location is to be used to avoid registration errors, the single ribbon associated with that one location may become very specialized. What is needed and would be useful, therefore, is a means for conducting in a consistent way various multiple image printing tasks wherein the substrate need not be moved between printing locations, and indeed for reasons of printing speed, wherein more than one printing process can be carried out at once. Accuracy of registration between images will thus be limited only by the printing process itself, and not by having been required to move the image-receiving substrate after having completed one printing task so as to commence another. Although at the expense of providing one or more additional print heads, that capability is provided by the present invention in which multiple ribbons and associated energy sources for image transfer are able to carry out successive printing operations on a substrate that has remained held throughout.